In an open letter on their website Apple have detailed the number and type of requests the US government have filed for users personal information. In very much the same fashion as other companies such as Microsoft and Facebook they say they had never heard of PRISM before the media reports and they do not give the government direct access to their servers.
In terms of numbers Apple say they have received between 4,000 and 5,000 requests in the six months from the start of December 2012 through to the end of May 2013. These requests relate to between 9,000 and 10,000 devices. Apple also say:

"The most common form of request comes from police investigating robberies and other crimes, searching for missing children, trying to locate a patient with Alzheimer’s disease, or hoping to prevent a suicide."

They make clear that certain information such as the content of iMessages and FaceTime conversations are not stored and/or passed on to authorities. A clear process is set out where a court order must be in place before a legal team then evaluates each request and some are even disregarded.

Apple finish by saying:

We will continue to work hard to strike the right balance between fulfilling our legal responsibilities and protecting our customers’ privacy as they expect and deserve.

The numbers seem pretty small especially when you consider the hundreds of millions of Apple devices out there.